The GaN/AlxGa(1-x)N HEMT (High Electron Mobility Transistor) concept is based on the formation of a 2-dimensional electron gas (2DEG) at the interface between GaN and AlxGa(1-x)N. In general, for the formation of a 2DEG, two semiconductor materials with similar lattice constants but different band gaps are needed. At the interface between the two semiconductors e.g. GaN and AlxGa(1-x)N, due to the different band gaps, there is a band bending phenomenon in which the conduction band energy minimum line (CB) of the large band gap semiconductor (e.g. AlGaN) is bent upwards, while the CB of the smaller band gap semiconductor (e.g. AlxGa(1-x)N) is bent downwards. This phenomenon leads to the formation of a triangular potential well at the interface. If the minimum of the potential well is lower than the Fermi energy of the material, the potential well is populated by electrons that will be confined in the z-direction, therefore forming a 2-dimensional system. The degree to which the potential well is populated by electrons can be tuned by engineering the band gap of the AlxGa(1-x)N.
A HEMT structure typically includes a substrate having a surface which supports epitaxial growth of Group III nitride-based layer such as a GaN channel layer and an AlxGa(1-x)N barrier layer. In the case of GaN HEMTs, the substrate can include many different multi epitaxial layer stacks such as consecutive AlxGa(1-x)N layers with increasing thickness and decreasing Al content, a superlattice such as alternating thin GaN and AlN layers, a structure with a back barrier layer like an additional AlxGa(1-x)N layer with very small Al content, etc. Each type of substrate can be very different in terms of number of layers, layer thicknesses and compositions.
The Al content in the AlxGa(1-x)N barrier layer of a GaN/AlxGa(1-x)N HEMT structure influences the electron density in the 2D electron gas (2DEG) and therefore defines the threshold voltage, breakthrough voltage, and other device parameters. The energy gap of AlxGa(1-x)N is directly proportional to Al concentration. Therefore, Al concentration in the barrier layer should be a very well defined parameter with a very narrow process tolerance. For example, in the case of some GaN/AlxGa(1-x)N HEMT devices, an epitaxial growth process tolerance of 1% is targeted for the Al content in the barrier layer.
As HEMT structures consist of crystalline layers, one way of measuring Al content in the barrier layer is by the use of HRXRD (High Resolution X-Ray Diffraction) methods. To precisely determine the Al content in the barrier layer of a HEMT structure, the lattice parameters may be determined first. In case of an epitaxial layer, lattice constants are defined, among others, by composition and strain or stress.
A conventional method commonly used is the so-called Omega-2Theta scans on 002, 004 and 006 reflections of a GaN/AlxGa(1-x)N HEMT structure. The interpretation or analysis of these scans may raises difficulties due to low intensity peaks and peak overlaps that may prevent precise determination of the lattice parameters and the necessary precision for determining the Al concentration in the barrier layer. For example, in symmetric scans, i.e. (002) it is not possible to distinguish if the shift in peak position is due to a change in strain levels or due to a change in composition.
Accordingly, there is a need for more precisely determining lattice parameters of the barrier layer of a HEMT structure.